In the wireless communications field, increasing use is frequently made of MIMO (Multiple Input Multiple Output) circuits in order to increase the capacity of the transmission circuits and improve the operation of the entire system. The use of MIMO circuits generally leads to an increase in the number of antennas to be realised for a single board. Moreover, to facilitate the integration of the circuits, the antennas are now produced directly on the printed circuit board or PCB. However, in application of the laws of physics, the length of an antenna is a function of the wavelength. Hence, to be able to operate in WiFi, that is for example in the frequency band of 2.4 GHz, the length of a slot antenna as a function of λg is several tens of millimeters. This length is not negligible when the antenna must be integrated on printed circuit boards used in mass production. Moreover, the printed circuit boards are most often constituted by substrates with a multilayer structure.
Hence, to produce a compact slot antenna using the multilayer structure of the substrate, the most natural idea consists in folding the slot-line in the manner shown in FIGS. 1 and 2.
In FIG. 1, a cross sectional view has been shown diagrammatically of a substrate with two dielectric layers d1, d2 and with three conductive layers M1, M2, M3. To produce a compact slot antenna in this type of substrate, a slot-line was etched successively in the conductive layer M3, as shown by the slot-line 1. Then, after passing through the dielectric layer d2, the slot-line continues by a slot-line 2 produced in the conductive layer M2. It then passes through the dielectric layer d1, and it continues by a slot-line 3 produced in the conductive layer M1. The supply point 4 of the slot antenna is formed at the level of the slot-line 1. This supply is realised in a standard manner by electromagnetic coupling, according to the technique known as “Knorr”. In this case, the three slot-lines 1, 2, 3 are superimposed and they have a total electrical length, between the supply point 4 and the short circuit extremity of the slot-line 3, equal to λg/2 where λg is the guided wavelength in the slot at the operating frequency.
A more detailed representation of a doubly folded slot antenna, such as the one in FIG. 1, is given by the perspective view of FIG. 2. In this case, only the parts of the conductive layers M1, M2, M3, necessary for a correct understanding of the invention, are shown. Hence, the slot-line 1 was etched in the lower conductive layer M3, this slot being in open circuit at one extremity, the other extremity not shown being coupled to the supply line. Moreover, a slot-line 2 was etched in the conductive layer M2 that is delimited by two conductive strips B2, B′2 that, in the embodiment shown, have an L-shape. Next, in the conductive layer M1, was produced a third slot-line 3 delimited by two conductive strips B3, B3′, also in an L-shape. The two conductive strips B3 and B3′ have on one side an extremity in short-circuit, as shown by the conductive strip B″3. Moreover, the conductive strips B3 and B2 are interconnected on the side of the supply point extremity by a via V1 itself connected to an isolated element of the conductive layer M3. Likewise, two conductive strips B′3, B′2 are connected to an isolated element of the conductive layer M3 by a via V′1.
Moreover, as shown in FIG. 2, the other opposite extremities of the strips B2 and B′2 delimiting the slot-line 2 in open circuit, are connected by vias V2 and V′2, respectively to the conductive layer M3 and to two isolated elements of the conductive layer M1 realised in the continuation of layers B3 and B′3. As shown in FIG. 2, the three slot-lines 1, 2, 3 are superimposed.
An antenna of this type whose electrical length of the three slot-elements 1, 2, 3 between the supply point and the open circuit extremity of the slot 3 is equal to λg/2, has been simulated for a WiFi operation, that is in the band of the 2.4 GHz. The simulation was made using the electromagnetic simulator Momentum d'Agilent, by using FR4 substrates as substrate with metallization levels spaced by 0.5 mm. In this case, the impedance matching curve as a function of the frequency is shown in FIG. 3 for a structure such as the one in FIGS. 1 and 2. This curve has a resonance at a frequency of 2.8 GHz, greater than the frequency of the WiFi band. Moreover, a secondary spurious resonance appears towards the 3.7 GHz, which denotes an atypical behaviour of the slot antenna resulting from such a stacking of slot-lines.